The anionic polymerization of lactams in molds has been used and studied for several decades. Three essential components are involved in the reaction, namely, the lactam (or polyamide monomer) to be polymerized, the catalyst and the initiator or activator.
Up until now there are basic ways for mixing and feeding the reactive mixture to a mold. In the first option two independent premixtures are prepared, the first one comprises a mixture of the catalyst and a fraction of the lactam; and the second one consists of the initiator and the remaining fraction of the lactam. Both premixtures are subsequently combined to carry out the polymerization. An example of this way of polymerization can be found in U.S. Pat. No. 3,505,448; U.S. Pat. No. 4,182,843; U.S. Pat. No. 4,233,433, and U.S. Pat. No. 4,404,360.
In the second option, a premixture of the catalytic system, i.e., a mixture of the catalyst and activator, is prepared to then mix the catalytic system with the lactam, and all these ingredients already mixed are taken to the mold in which they are polymerized, examples of this way of polymerization of lactams are in patents EP 0,872,508; U.S. Pat. No. 5,747,634 and U.S. Pat. No. 5,756,647.
In fact, the latest efforts in improving the technique of polymerization of lactams have been focused on modifying the catalytic system to carry out the polymerization in a controlled and selective manner. However, the way in which the basic ingredients are mixed and fed to the mold has not changed and is still basically one of the two forms described above, i.e., the general technique is measuring the amounts of activator and catalyst separately and after preparing the respective premixtures.
Both traditional techniques have associated therewith high residence temperatures and times for the premixtures in the tanks. Obviously, in addition to the tanks or reservoirs necessary for each of the components for the polymerization, tanks or reservoirs are needed which are heated or maintained in special conditions for preparing the premixtures and having them at the suitable temperature before being injected to a mixing head so that from there they pass to the mold. For example in patent U.S. Pat. No. 4,404,360 a pair of tanks are used for the premixtures.
All the previous techniques allow producing serial parts of one and the same composition and properties but they are not suitable in the continuous manufacture of non-serial parts, for example with individual sizes and/or geometries, different compositions or characteristics.